Electrolytic processes are practiced on a large scale throughout the world. For example, there are many industrial processes in which water or an aqueous solution is electrolyzed, for example, an aqueous solution of an acid or an aqueous solution of an alkali metal chloride. Aqueous acidic solutions are electrolyzed in, for example, electrowinning, electrotinning and electrogalvanizing processes, and aqueous alkali metal chloride solutions are electrolyzed in the production of chlorine and alkali-metal hydroxide, alkali metal hypochlorite, and alkali metal chlorate. The production of chlorine and alkali metal hydroxide is practiced in electrolytic cells which comprise a mercury cathode or in electrolytic cells which comprise a plurality of alternating anodes and cathodes, which are generally of foraminate structure, arranged in separate anode and cathode compartments. These latter cells also comprise a separator, which may be a hydraulically permeable porous diaphragm or a substantially hydraulically impermeable ion-exchange membrane, positioned between adjacent anodes and cathodes thereby separating the anode compartments from the cathode compartments, and the cells are also equipped with means for feeding electrolyte to the anode compartments and if necessary liquid to the cathode compartments, and with means for removing the products of electrolysis from these compartments. In a cell equipped with a porous diaphragm, aqueous alkali metal chloride solution is charged to the anode compartments of the cell, and chlorine is discharged from the anode compartments and hydrogen and cell liquor containing alkali metal hydroxide are discharged from the cathode compartments of the cell. In a cell equipped with an ion-exchange membrane aqueous alkali metal chloride solution is charged to the anode compartments of the cell and water or dilute aqueous alkali metal hydroxide solution to the cathode compartments of the cell, and chlorine and depleted aqueous alkali metal chloride solution are discharged from the anode compartments of the cell and hydrogen and alkali metal hydroxide are discharged from the cathode compartments of the cell.
Electrolytic cells are also used in the electrolysis of non-aqueous electrolytes and in electrosynthesis.
It is desirable to operate such electrolytic cells at as low a voltage as possible in order to consume as little electrical power as possible and in such a way that the component parts of the electrolytic cell are long lasting, i.e. the electrodes in the electrolytic cell should have a long lifetime.
In recent years anodes which have been used in such electrolytic processes have comprised a substrate of titanium or of an alloy of titanium possessing properties similar to those of titanium and a coating of an electrocatalytically-active material on the surface of the substrate. An uncoated titanium anode could not be used in such an electrolytic process as the surface of the titanium would oxidize when anodically polarized and the titanium would soon cease to function as an anode. The use of such a coating of electrocatalytically-active material is essential in order that the titanium shall continue to function as an anode. Examples of such electrocatalytically-active materials which have been used include metals of the platinum group, oxides of metals of the platinum group, mixtures of one or more such metals and one or more such oxides, and mixtures or solid solutions of one or more oxides of a platinum group metal and tin oxide or one or more oxides of a valve metal, that is one or more oxides of titanium, tantalum, zirconium, niobium, hafnium or tungsten.
Recently it has been suggested in EP 0,437,178 that anodes wherein the coating comprises mixed oxides of iridium, ruthenium and titanium having oxide molar ratios of Ti:(Ir+Ru) of less than 1:1 and of Ru:Ir of between 1.5:1 and 3:1 can be prepared from a certain acidic aqueous solution.
Likewise, it has been suggested in J 59,064788 that electrode coatings can be prepared by the deposition of certain coatings from organic solvents onto a substrate followed by heating the coated substrate in oxygen.